The basic design of balloon catheters as known from documents U.S. Pat. No. 5,522,882 A and U.S. Pat. No. 7,217,278 B2, for example, is stated in the preamble of claim 1. Balloon catheters as used, for example, for expanding abnormally constricted blood vessels in the body or for placing vessel wall supports (referred to as “stents”) have an outer shaft with a distal end and an inner shaft, situated therein to form an annular fluid line, projecting beyond the distal end of the outer shaft. At the distal end of the catheter a balloon is attached at its proximal end in a fluid-tight manner to the distal end region of the outer shaft at a first attachment zone, and at its distal end is attached in a fluid-tight manner to the distal end region of the inner shaft at a second attachment zone. Between these attachment zones, in the undilated state the balloon is placed in longitudinal folds in order to minimize its outer diameter in this state. This is necessary to allow the balloon catheter together with the balloon to be pushed at the distal end through narrow vessels or sharply curved vessel regions. After the balloon is set in position at the application site, a fluid under pressure may be introduced through the annular fluid line formed between the inner and outer shafts, and the balloon may be dilated. This causes the longitudinal folds to unfold in the peripheral direction, with a large increase in the diameter of the balloon.
Because of their manufacturing method and design, the balloons of conventional balloon catheters have disadvantages which will become apparent from the following summary of the production process. Balloons are generally manufactured from a plastically distendable plastic tube having an outer diameter of 2.1 mm, for example, and a lumen diameter of 1.5 mm, for example. The wall thickness of this tube is thus 0.3 mm. The ends of the tube are clamped into a holding device, whereupon the lumen is acted on by a fluid pressure.
Between the clamping points the workpiece is inflated and the wall material is drastically stretched, resulting in an essentially cylindrical balloon having a wall thickness of 0.03 mm, for example. From the clamped ends of the balloon preform the wall thickness decreases by a factor of 10, for example, over the tapers at the two ends of the balloon toward the shell wall.
In a further processing step the ends are sized and brought to an outer diameter of 1.8 mm and a lumen diameter of 1.6 mm, for example. The wall thickness is then 0.1 mm, and therefore is still greater than three times the wall thickness in the cylindrical portion of the balloon preform.
When balloons manufactured in this manner are then attached at their ends to the outer and inner shafts of the catheter and longitudinally folded for the undilated state, the ends and tapers of the balloon having a much greater wall thickness are built up much more than the very thin-walled cylindrical shell of the balloon. The folded balloon profile is therefore greatest around the attachment zones at the tapers which form the balloon shoulders. Correspondingly, the balloon also has the greatest stiffness at that location. These greatly built-up end regions of the balloon thus prevent insertion of the catheter into narrow blood vessels. The stiffness of the balloon tapers placed into folds also makes it more difficult to guide the balloon catheter around narrow curvatures or branches of vessels. Lastly, during manufacture of the balloon catheter itself it is difficult to fold the tapered regions having a greater wall thickness.
U.S. Pat. No. 5,522,882 A mentioned above discloses a stent positioning system which includes a catheter in which the dilatable balloon has ends which extend in a stepwise manner. The aim is to avoid the presence of tapered sections of the balloon in the axial direction before and after the stent which is positioned on the catheter. This is achieved by use of sleeve-like attachments which directly adjoin the stent on the end regions of the balloon, so that when the balloon is dilated the tapers are essentially provided as radial annular steps. The problems of the differing wall thicknesses and of folding for conventional balloons are not addressed in this document.
U.S. Pat. No. 7,217,278 B2 teaches a complex finishing of balloon preforms, in that after inflation, wall material is mechanically removed in the region of the tapers and thick-walled ends in order to reduce the wall thickness. In this regard particular care must be taken that this processing step is carried out below the glass transition temperature of the thermoplastic plastic material. Thus, on the whole the manufacture of balloons such as those known from U.S. Pat. No. 7,217,278 B2 is complicated from a production standpoint.